1. Field of the Invention
The present invention relates to a pellicle used in a step of photolithography in the production of integrated circuits for the purpose of preventing dust and dirt from adhering onto the mask or the reticle (hereinafter simply referred to as mask and the like). More specifically, the invention relates to a pellicle of which an adhesive resin layer for mounting the pellicle on the mask is formed of a hard resin layer and a soft resin layer.
2. Description of the Prior Art
In the above-mentioned pellicle, in general, a transparent pellicle film of a nitrocellulose or the like is stretched, via an adhesive layer, on an end of a pellicle frame made of aluminum or the like, and the other end of the pellicle frame is adhered with an adhesive (mask-adhering) layer onto the pattern-forming surface of the mask or the like, so that the pellicle will not be removed from the mask or the like during the handling.
When the mask-adhering agent exhibits a too strong adhering force, therefore, the mask is damaged due to the application of an excessive force at the time of peeling the pellicle off the mask and the like or the mask-adhering agent remains on the mask after it has been peeled off. As a result, the semiconductor devices could become defective or the yield of production decreases.
From such a point of view, there have been proposed a pellicle with a mask-adhering agent exhibiting a weaker adhering force to the mask than to the pellicle (Japanese Unexamined Patent Publication (Kokai) No. 75835/1985), and a pellicle using a mask-adhering agent having a hardness of not larger than 200 gf (Japanese Unexamined Patent Publication (Kokai) No. 282640/1998).
Despite the mask-adhering layer formed between the mask and the pellicle frame has an adhering force as taught in the above prior art (Japanese Unexamined Patent Publication (Kokai) No. 75835/1985), however, the mask is distorted and loses flatness. That is, when the mask loses flatness, the pattern formed on the mask is distorted, and a correct pattern is not transferred onto the wafer.
In the step of photolithography, furthermore, exposure to light is effected not only where flat surfaces of the mask and of the pellicle film are maintained in a horizontal direction but also where flat surfaces of the mask and of the pellicle film are maintained in a vertical direction. When the surface of the mask is maintained in the vertical direction, however, the layer of the mask-adhering layer hangs down due to its own weight in the case of the mask-adhering agent taught in the above Japanese Unexamined Patent Publication (Kokai) No. 282640/1998 spoiling the flatness of the mask.
It is therefore an object of the present invention to provide a pellicle which gives no damage to the mask, which can be mounted on, and removed from, the mask, and which does not lose flatness of the mask even when the mask surface is maintained in either the horizontal direction or the vertical direction.
According to the present invention, there is provided a pellicle comprising a pellicle film, a pellicle frame supporting the pellicle film, and an adhesive resin layer provided on the surface of the pellicle frame on the side opposite to the surface supporting the pellicle film, wherein the adhesive resin layer is formed of a combination of a hard resin layer and a soft resin layer.
In the pellicle of the present invention, it is desired that:
1. The hard resin has a hardness (JIS A) of not smaller than 170 gf and the soft resin has a hardness (JIS A) of not larger than 100 gf; and
2. The soft resin layer is formed maintaining a thickness larger than that of the hard resin layer.
Further, the adhesive resin layer has such a layer structure that the soft resin layer is located on the side of the pellicle frame and the hard resin layer is located on the soft resin layer. With this layer structure, the pellicle is mounted on the mask via the hard resin layer. In this adhesive resin layer, it is desired that the hard resin layer has a thickness which is from 5 to 30% of the thickness of the adhesive resin layer, and the soft resin layer has a thickness which is from 95 to 70% of the thickness of the adhesive resin layer. Further, the soft resin layer can be sandwiched between the two hard resin layers. In this case, the hard resin layer comes in contact with both the pellicle frame and the mask.
Further, the adhesive resin layer can have such a layer structure that both the soft resin layer and the hard resin layer come in contact with the pellicle frame. With this layer structure, both the soft resin layer and the hard resin layer come in contact with the mask that is mounted. In this adhesive resin layer, it is desired that the hard resin layer is disposed on the lower side of the soft resin layer. There can be further employed such a layer structure that the soft resin layer is disposed between the two hard resin layers. It is further desired that the width of contact between the hard resin layer and the mask that is mounted, is from 10 to 30% of the width of contact between the adhesive resin layer and the mask that is mounted.